TC826 datasheet(10/18 Pages) MICROCHIP

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TC826

DS21477B-page 10

4.2

System Timing

The oscillator frequency is divided by 32 prior to clock-

ing the internal counters. The three-phase measure-

ment cycle takes a total of 80 clock pulses. The 80

count cycle is independent of input signal magnitude.

Each phase of the measurement cycle has the follow-

ing length:

• Auto-Zero Phase: 19 to 59 Counts

For signals less than full scale, the auto-zero is

assigned the unused reference integrate time period.

• Signal Integrate: 20 Counts

This time period is fixed. The integration period is:

EQUATION 4-3:

• Reference Integrate: 0 to 41 Counts

4.3

Reference Voltage Selection

A full scale reading requires the input signal be twice

the reference voltage. The reference potential is mea-

sured between REF

(Pin

5) and ANALOG

COMMON (Pin 2).

TABLE 4-1:

The internal voltage reference potential available at

analog common will normally be used to supply the

converter’s reference. This potential is stable when-

ever the supply potential is greater than approximately

7V. In applications where an externally generated refer-

ence voltage is desired, refer to Figure 4-3.

The reference voltage is adjusted with a near full scale

input signal. Adjust for proper LCD display read out.

FIGURE 4-3:

EXTERNAL REFERENCE

4.4

Components Value Selection

4.4.1

INTEGRATING RESISTOR (RINT)

The desired full scale input voltage and output current

capability of the input buffer and integrator amplifier set

the integration resistor value. The internal class A out-

put stage amplifiers will supply a 1

µA drive current with

minimal linearity error. RINT is easily calculated for a

µA full scale current:

EQUATION 4-4:

4.4.2

INTEGRATING CAPACITOR (CINT)

The integrating capacitor should be selected to maxi-

mize integrator output swing. The integrator output will

swingtowithin0.4V of VS+or VS- without saturating.

The integrating capacitor is easily calculated:

EQUATION 4-5:

The integrating capacitor should be selected for low

dielectric absorption to prevent rollover errors. Polypro-

pylene capacitors are suggested.

4.4.3

AUTO-ZERO CAPACITOR (CAZ)

CAZ should be 2-3 times larger than the integration

capacitor. A polypropylene capacitor is suggested. Typ-

ical values from 0.14

µF to 0.068µFare satisfactory.

4.4.4

REFERENCE CAPACITOR (CREF)

µF capacitor is suggested. Low leakage capacitors,

such as polypropylene, are recommended.

Several capacitor/resistor combinations for common

full scale input conditions are given in Table 4-2.

Required Full Scale Voltage

VREF

20mV

10mV

TSI =20









FOSC

Where FOSC is the externally set clock frequency.

TC826

REF IN

ANALOG

COMMON

2.50V

Reference

V+

(b)

MCP1525

µF

Where VFS = Full Scale Analog Input

RINT =

Full Scale Voltage(V)

1x10–6

VFS

1x10 – 6

Where: VINT = Integrator Swing

FOSC = Oscillator Frequency

CINT =

VFS

RINT









640

FOSC xVINT

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